In OFDM data transmission systems such as the downlink of the UMTS LTE, data may be transmitted in either localised or distributed modes. In localised mode, data for one user is transmitted using a contiguous block of OFDM sub-carriers, while in distributed mode data is transmitted on a plurality of separated sub-carriers.
Typically, data for transmission is subdivided into data blocks known as Virtual Resource Blocks (VRB). The data is then mapped onto specific resource elements on the OFDM waveform, where one resource element comprises one OFDM symbol duration on one sub-carrier. The resource elements of the OFDM waveform are grouped into Physical Resource Blocks (PRBs) which may for example comprise 12 sub-carriers for a duration of 14 OFDM symbols.
The diagram of FIG. 1 defines the terminology: a VRB may typically contain the same amount of data as can be mapped to a PRB. VRBs containing data for mapping to the PRBs in a localised way are known as Localised VRBs (LVRBs), while VRBs containing data for mapping to the PRBs in a distributed way are known as Distributed VRBs (DVRBs).
Some known ways of mapping DVRBs to PRBs include:
Fully-scattered mapping illustrated on FIG. 2: This involves placing individual data symbols from the DVRBs into scattered (possibly pseudo-random) positions in the PRBs containing data from LVRBs. This has the disadvantage that the users receiving the localised transmissions need to know the locations of the individual distributed data symbols and remove these from their own data. On the other hand, a fully-scattered mapping maximises transmit diversity for the distributed data, and also helps to randomise inter-cell interference in a cellular communication system.
Grouped mapping is illustrated on FIG. 3. Here each PRB contains data either from a LVRB or from a DVRB but not both. This avoids the drawback of the localised data receivers needing to know the locations of distributed data symbols. However, the degree of diversity for the distributed transmissions is reduced, especially if the data from one DVRB is mapped onto specific subcarriers for the duration of a subframe as shown above.
As a consequence, there is a need for an improved method allowing a large frequency diversity of the mapping.